🚀 go-pugleaf

Hi Netland...

I have a little problem that I can't solve immediately, coz I havent'
got/can't find the relevant text book.

Here is the outlay... I have two (later maybe more) audio signals, that
are amplitude limited to 2V peak to peak, but no low-frequency
limitations.  (Eg DC to 20kHz).  I want to "multiply" the two signals
together, which, if I remember rightly, is the same as modulating one
signal with the other.  Since modulation is symmetric, if 100%
modulation is used, I figure that is the same as multiplying them
together.  I know there is an op-amp circuit somewhere to do exactly
what I want, but I can't remember, (it was 8 years since I started
Engineering, and I haven't used much of it since.  Poor old memory...)

The only tricky bits are:
1) if _either_ one of the inputs is 0v (i.e. no signal), then the output
must be as close to 0 as possible.  I seem to remember some modulation
circuits that still let one signal through at about 10% for zero
modulation input.

2) Ideally, I should be able to switch the inputs over (i.e. swap roles
from "carrier" to "modulation") with no discernible difference in
output.  This isn't totally necessary, I will probably end up with a
computer controlled soft-switcher to swap signals around later, but it
would be nice in the early stages.

3) If possible, it should be easily expanded to handle more than two
inputs, hopefully without cascading, coz I don't want too many booster
amplifiers to bring the signal back up to 2v after every stage.


Can anyone remember their op-amp theory, or alternatively suggest a
good, _current_ electronics text that covers this sort of thing?

Please respond to email address below, I don't think too many others
will want to wade through this stuff!

Thanks in advance.


[dav]    aka
David Emrich
gude@uniwa.uwa.edu.au